refactor: enable compile-time D-Bus signature generation (#425)

Since sdbus-c++ knows types of D-Bus call/signal/property input/output parameters at compile time, why not assemble the D-Bus signature strings at compile time, too, instead of assembling them at run time as std::string with likely cost of (unnecessary) heap allocations...

std::array is well supported constexpr type in C++20, so we harness it to build the D-Bus signature string and store it into the binary at compile time.

tests/unittests/Message_test.cpp

@@ -51,7 +51,7 @@ namespace sdbus {
     template <typename _ElementType>
     sdbus::Message& operator<<(sdbus::Message& msg, const std::list<_ElementType>& items)
     {
-        msg.openContainer(sdbus::signature_of<_ElementType>::str());
+        msg.openContainer<_ElementType>();
 
         for (const auto& item : items)
             msg << item;
@@ -64,7 +64,7 @@ namespace sdbus {
     template <typename _ElementType>
     sdbus::Message& operator>>(sdbus::Message& msg, std::list<_ElementType>& items)
     {
-        if(!msg.enterContainer(sdbus::signature_of<_ElementType>::str()))
+        if(!msg.enterContainer<_ElementType>())
             return msg;
 
         while (true)

tests/unittests/TypeTraits_test.cpp

@@ -170,7 +170,8 @@ namespace
 
 TYPED_TEST(Type2DBusTypeSignatureConversion, ConvertsTypeToProperDBusSignature)
 {
-    ASSERT_THAT(sdbus::signature_of<TypeParam>::str(), Eq(this->dbusTypeSignature_));
+    constexpr auto signature = sdbus::as_null_terminated(sdbus::signature_of_v<TypeParam>);
+    ASSERT_THAT(signature.data(), Eq(this->dbusTypeSignature_));
 }
 
 TEST(FreeFunctionTypeTraits, DetectsTraitsOfTrivialSignatureFunction)